Power converters that employ self-driven synchronous rectifiers are becoming increasingly popular in the power industry today, because of their compact size and high efficiency. For instance, one particular class of self-driven synchronous rectifier circuits employing active clamp topologies may attain conversion efficiencies exceeding 90%, (e.g., a 48 V to 5 V DC-to-DC converter).
Nevertheless, the efficiency of self-driven synchronous rectifiers, are often substantially reduced by non-idealized parasitic elements, such as: (1) stray inductances introduced by component interconnections, (2) transformer leakage inductances, (3) body capacitances of the synchronous rectifiers, (4) and other related parasitic phenomena. Such parasitic elements form high frequency resonant circuits which can oscillate (generating a "ringing") as the synchronous rectifier gates transition between ON-and-OFF and OFF-and-ON. As result, during transition periods, the ringing creates a momentary short circuit of the synchronous rectifiers, allowing the synchronous rectifier to simultaneously "cross conduct" for brief periods of time. Such simultaneous conduction (i.e., cross conduction) can be a significant source of power loss and inefficiency for self-driven synchronous rectifiers.
Cross conduction and ringing is becoming more troublesome as power converter designs migrate to very low output voltages (e.g., 1.5 V to 2 V). Low output voltage converters often require the use of low-threshold-voltage type Metal Oxide Silicon Field Effect Transistors (MOSFETs) (typically less than &lt;2.5 volts), because voltages available at the winding terminals of such transformers are no longer sufficient to adequately drive traditional power MOSFETs, (e.g., 10 volt gate drive). Unfortunately, low threshold MOSFETs are considerably more susceptible to cross conduction than conventional 10 volt drive power MOSFETs.
What is needed, therefore, is way to substantially eliminate cross conduction of rectifiers in self-driven synchronous converters without introducing unnecessary circuit complexity or additional parasitic loss mechanisms.